A high precision and high speed rotation have increasingly been requested, heretofore, in a spindle motor used in a hard disk drive along with realization of a higher density and higher speed data transmission in the hard disk drive. In recent years, a dynamic pressure oil bearing motor has been employed as a technique of serving for such a request. There has also been studied a gas dynamic bearing motor capable of a high speed rotation without scattering of oil or degradation of oil.
A gas has a viscosity lower than oil and a compressible fluid; therefore, a gas dynamic bearing has a low load capacity per a unit area of the bearing, which inevitably requires a larger scale as compared with a dynamic oil bearing. If a gas dynamic bearing was constructed within a small motor used for a spindle of a hard disk drive, it would be hard to obtain a sufficient bearing stiffness. Hence, since a diameter of a radial bearing necessary to be the largest possible value, various constructions have been investigated in order to enhance a bearing stiffness.
In FIG. 17, there is shown a sectional view of an example of a conventional gas dynamic bearing. A stationary shaft 91 is constructed of a cylindrical shaft 1 and a ring-shaped thrust plate 2 fixed on the upper end of the shaft 1, wherein the lower end of the stationary shaft 91 is fixed to a base 7a. A lidded cylindrical hub 3d is fitted with an outer circumferential surface 81 of the shaft 1 and an upper surface 85 of the thrust plate 2 so as to surround them with a predetermined clearance interposed therebetween. Inserted and fixed in a central hole of the hub 3d is a pin 4 with a magnet 5 fixed at the lower portion thereof and a flange 41 in the middle portion thereof, and a rotor assembly 92 is constructed of the hub 3d, the pin 4 and the magnet 5 combined. A flange-shaped disk receiving portion 31d extending outwardly in a radial direction is provided at the lower end of the hub 3d, on which recording disks 9 and spacers 10 are loaded and fixed by screwing a clamp 8 at the top thereof into the hub 3d. 
Pressure generating grooves (not shown) are provided on the outer circumferential surface 81 of the shaft 1 to thereby form a radial bearing between an inner circumferential 82d of the hub 3d and the outer circumferential surface 81. Pressure generating grooves (not shown) are provided on the upper surface 85 and lower surface 84 of the thrust plate 2, a first thrust bearing is constructed of the upper surface 85 of the thrust plate 2 and an upper surface 86 inside the hub 3d and a second thrust bearing is constructed of the lower surface 84 of the thrust plate 2 and the upper surface 83 of the flange 41 of the pin 4.
A stator core assembly 6 is fixed to the base 7a opposite the magnet 5 and a current is supplied through the stator core assembly 6 to thereby rotate the recording disks 9 together with the rotor assembly 92. With rotation of the rotor assembly 92, dynamic pressures are generated in the radial bearing, the first and second thrust bearings, which makes it possible to hold the rotor assembly 92 in a non-contact state.
With a construction similar to the above case, a technique has been adopted in which provided in a radial bearing are pressure generating grooves forcibly sending a gas in a thrust bearing direction, while provided in a thrust bearing are pressure generating grooves forcibly sending the gas in an outer circumference direction, so as to obtain a sufficient thrust load capacity (see JP 2002-238212 A, for example).
A technique has been adopted in which a thrust dynamic pressure oil bearing is provided inside a radial dynamic gas bearing to thereby make the motor compact and set a diameter of the radial bearing large, thereby enabling a sufficient radial bearing stiffness to be obtained (see JP 2800278 B and JP 2000-179542 A, for example).
A technique has been adopted in which a stationary shaft is constructed so as to be separated into a first portion on the base side and a second portion on the distal end side to thereby set the diameter of a radial bearing to the largest possible value, thereby enabling a sufficient radial bearing stiffness to be obtained (see JP 2000-50568 A, for example).
In a gas dynamic bearing motor as in the conventional examples described above, a diameter of a radial bearing has been set to the largest possible value in order to obtain a sufficient radial bearing stiffness; therefore, there has been a trend toward a smaller thickness of the hub 3d, which is a difference in radius between the inner circumferential surface and outer circumferential surface of the hub 3d. Consequently, a large force acts on a disk receiving portion when the recording disks 9 are loaded on the hub 3d and fixed with the clamp 8; therefore, the hub 3d deforms so that the lower end portion of the inner circumferential surface of the hub 3d is bent inwardly.
In a conventional gas dynamic bearing motor, machining of parts has been made so that the outer circumferential surface of the shaft 1 and the inner circumferential surface of the hub 3d face each other with a predetermined clearance interposed therebetween in the state where no recording disk 9 is loaded on the hub 3d. Therefore, the clearance between the outer circumferential surface of the shaft 1 and the inner circumferential surface of the hub 3d becomes narrower in the lower end portion of the radial bearing when the recording disks 9 are loaded on the hub 3d for assembly, resulting in occurrence of partial contact, generation of abrasive wear and lock of the bearing.
Wear powder generated by friction on bearing surfaces scatters outside the motor through a clearance between the base and the lower end surface of the hub to contaminate the interior of the hard disk drive.